Why does it state "Do not reduce pipe size"

I bought a house, it's on a share well. The well pumps water into a holding tank. Everybody has separate lines running from the holding tank to their homes.
At the home, I have a pressure tank and flowtec pump to pull water from the holding tank.

I want to replace both the pump and pressure tank. (Leaking and suspecting the bladder in the pressure tank is leaking)

Factory Advice for Pressure Tank and Pump Plumbing:

The pressure tank has 1 and quarter inch fitting.
The pump has a 1 and quarter inch inlet and a 1 inch outlet.

The manual states "These sizes should not be reduced".

Plumbing from storage tank to house:

The PVC pipe from the storage tank to the house runs about 150ft and is 1 inch. This line is then reduced to 3/4 inch copper pipe. This 3/4 inch copper is the home's core plumbing size (it's a manufacture home).

Current plumbing at the pressure tank and pump:

The current pressure tank and pump are located next to the house. At this location, there's a "from storage tank" line and a "to house" line both are 3/4 inch copper.

The current (old) system has the 3/4 inch copper line "from the storage tank" which is increased to 1 inch. Then the pump intake has the 1 and quarter inch reduced to 1 inch (Fitting).

The pressure tank has the 1 and quarter reduced to 1 inch (fittings), then 1 inch pipe to the 1 inch pump outlet fitting. Then runs as 1 inch pipe, which is further reduced to a 3/4 inch copper line "to the house".

The current system has ran with the above plumbing setup for at least 5 years with no issues. I also believe that the pressure tank is 7 years old according to the previous owner.

Questions:

Should I replace the plumbing between the pump and pressure tank with the recommended pipe size even though the "from storage tank" and "to house" copper lines are 3/4 inch?

Or

Can I run 3/4 inch plumbing between everything? What are the long term or short term problems I could encounter?

Sorry for so long, but I'm trying to cover everything in detail.
House has 2 baths, pressure tank is 50 gallons, pump is 3/4 hp, and only 2 people live in the home.

I would pay attenton to what Terry said. However I would also suggest that if you are replacing the pump and the tank (if the tank is seven years old), consider installing a Pside-Kick www.cyclestopvalves.com with your new pump. This will give you a constant pressure while drawing a variable flow (like city water pressure). You're family will love it.